Adsorption Kinetics of Polymer/Surfactant Mixtures at an Expanding Air/Water Interface

Abstract

The dynamic adsorption mechanisms of a range of polymer/surfactant mixtures have been studied at
the expanding air/water interface created by an overflowing cylinder. The composition of the
adsorption layer from mixed systems is obtained using a new approach, co-modelling ellipsometry data
and NR data recorded on only one isotopic contrast, without deuterated polymer. The precision and
accuracy of the interfacial compositions using this novel approach match those obtained by NR
measurements using multiple isotopic contrasts and deuterated polymer, and exceeds those in the
absence of deuterated polymer.
For weakly interacting PEO/surfactant mixtures adsorption is competitive, the interfacial composition
can be rationalised in terms of competitive adsorption. At high surfactant concentrations polymer
adsorption is inhibited by the increasing surfactant coverage, although in PEO/SDS mixtures positive
interactions between the two components allow PEO to adsorb until an SDS monolayer is present.
For oppositely charged mixtures of PSS and CnTAB surfactants, synergistic adsorption occurs at low
surfactant concentrations, and the formation of polymer/surfactant complexes has a marked effect on
interfacial adsorption, although polymer adsorption is controlled by free polymer molecules.
Aggregation occurs around charge neutrality, the material in these aggregates cannot reach the
interface due to their size, and at higher surfactant concentrations polymer can no longer adsorb.
Mixtures of PEI/SDS at high pH behave similarly to the PSS/CnTAB systems, with progressive
aggregation depleting the system of surface active material and limiting adsorption. However at low
pH the aggregates can reach the interface by convection where they spread material across the surface
in the form of a thin layer of nanometer thickness by Marangoni flows.
This work proves that examination of the dynamic adsorption behaviour of polymer/surfactant systems
is invaluable to understanding their adsorption mechanisms. Furthermore there is a clear and
incontrovertible link between the dynamic interfacial adsorption and bulk phase behaviour.